Re: [Qemu-devel] buildbot failure in qemu on default_x86_64_rhel5

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Oct 04, 2011 at 02:18:20PM +0530, Aneesh Kumar K.V wrote:
> On Tue, 4 Oct 2011 08:18:07 +0100, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Mon, Oct 03, 2011 at 04:10:50PM +0530, Aneesh Kumar K.V wrote:
> > > On Mon, 3 Oct 2011 08:37:52 +0100, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > > > On Sat, Oct 01, 2011 at 03:03:23PM +0530, Aneesh Kumar K.V wrote:
> > > > > +#ifndef CONFIG_UTIMENSAT
> > > > > +    /*
> > > > > +     * We support handle fs driver only if all related
> > > > > +     * syscalls are provided by host.
> > > > > +     */
> > > > 
> > > > Perhaps a ./configure check should be added to see whether the handle
> > > > syscalls are supported instead of using CONFIG_UTIMENSAT.
> > > > 
> > > 
> > > We already do check for handle syscall. Since glibc doesn't have the
> > > this syscall yet, I added the check in virtio-9p-handle.c as below
> > 
> > CONFIG_UTIMENSAT is defined when the host has glibc >= 2.6.
> > 
> > Handle syscalls are available on Linux 2.6.39 but not exposed by glibc.
> > 
> > Therefore CONFIG_UTIMENSAT has nothing to do with handle syscalls and
> > does not mean handle syscalls are supported.  I would drop that hunk of
> > the patch or test for the actual handle syscalls in ./configure.
> 
> Here is what i am trying to achieve with the patch. For handle based fs
> driver to work we need to have 
> 
> 1) support for handle syscall
> 2) support for fd based syscalls like futimens, fstatat, readlinkat,
> fchmod, fchownat, openat etc.
> 
> Now handle syscalls are recently added to kernel and glibc doesn't have
> support for that. So what we did is to add handle syscall in
> virtio-9p-handle.c via syscall(2). Now if the syscall is not supported
> by the host kernel we will get ENOSYS. I only added support for i386 and
> x86_64, because most the syscall number varies with different archs. For
> other archs the wrapper returns ENOSYS. So instead of checking for
> handle syscalls in configure script we did the above to make sure it
> work without failure in most of the case. Once we have glibc support for
> handle syscall the above changes should be dropped in favor of
> configure script test.
> 
> Now for the fd based syscall dependency, we didn't initially had any
> check for that because my expectation was most glibc should
> have support for them. But RHEL 5 build failure indicate that futimens
> is not supported. We were already checking for futimens in configure so
> i added changes to make sure if futimens is not supported
> handle_utimensat returns error. (That was not added as a run time
> check, but rather a compile error fix). Now should we allow handle based fs
> driver if futimens is not supported. I was suggesting we should not;
> hence the check in init to return error when we don't support
> futimens. The later part of init routine do check whether handle
> syscalls are supported and disable handle fs driver if they are not.

Okay, then the comment should be:

/*
 * We support handle fs driver only if futimens is provided by the host
 */

The scenario where it might be possible to hit the CONFIG_UTIMENSAT is
with a modern kernel paired with an old userspace.  The handle syscall
which we call directly would succeed but the futimens(2) would not be
available.

On a sane system the handle syscall fails because the kernel doesn't
support it (and futimens isn't there either).

Stefan